The term “fastener” is used herein to describe rivets, screws, slugs or other types of fastening devices.
It is well known to distribute rivets or other fasteners (e.g. studs, nuts, bolts) between a supply and a setting tool of a riveting machine or setting tooling within a press by conveying them individually in delivery tubes by compressed air propulsion. A delivery tube of this kind typically has an internal passageway that is configured to conform closely to the shape of the fastener. Additional compressed air may be admitted into the tube at one or more locations along the length of the tube by means, for example, of T-junctions, branch tubes, or elbows connected between a supply of compressed air and the delivery tube, to propel the rivets along the length of the tube.
There are several problems associated with compressed air delivery tubes of the kind described above. First, there is a tendency for fasteners to jam in the tube. This renders the tube inoperable until steps have been taken to clear the blockage. It is often desirable to be able to transport a plurality of fasteners at once along a delivery tube but this significantly increases the tendency to jam. Secondly friction between the fasteners and the tubes causes tube wear. Delivery tubes have to be inspected at regular intervals for wear as a worn tube may be susceptible to blockages or slower feed rates causing increased cycle times. Moreover, wear often results in accumulation in the delivery tube of external dirt, dust or moisture or trace particles of the fastener, fastener coating or tube and they may also cause blockages and slower feed rates.
Compressed air pressure normally decreases along the length of the delivery tube owing to losses (such as for example leaks or friction losses). The desired pressure is generally maintained by supplying further air at booster points (in the form of T-junctions and branch tubes etc.) along the length of the tube and/or by increasing the pressure of the compressed air at source. The increase in air pressure and the addition of booster points increases air turbulence within the tube. If the flow of air is not as smooth as possible there is an increased tendency for fastener jams and the resultant scraping contact can in turn cause wear in the tube. The decreasing air-pressure owing to losses can also cause heavier fasteners to lose momentum so that they fail to reach the end of their journey with sufficient speed unless compressed air booster points are employed.
Existing delivery tubes are often lubricated to ensure that fasteners are transferred at acceptable speeds, and to reduce wear and friction. However, over a period of use the ingress of dirt can result in the tube becoming clogged by a paste comprising the combination of the dirt or trace particles and lubrication oil or moisture.
Known compressed air fastener delivery systems operate at an air pressure of between 5 and 6 bar. This pressure is normally required to ensure sufficient fastener momentum throughout the length of the delivery tube so that it reaches the desired destination despite friction losses or minor obstacles (dirt etc.) in the tube, and/or to accommodate air loss through leakages. A further disadvantage is that known systems may require the continuous application of such air pressure at 5–6 bar to maintain effective performance, as opposed to controlled blasts. Moreover, owing to the above limitations, known systems cannot reliably feed fasteners over long distances, the maximum possible tube lengths being typically 8 m–15 m.
It is an object of the present invention to obviate or mitigate the aforesaid disadvantages and to provide for fastener delivery apparatus that operates at higher transportation speeds, greater distances (typically in excess of 100 m) and lower air pressures (and therefore lower air consumption rate and cost) than currently possible without the use of a lubricant.